feat(webapp): add dashboard platform notifications service & UI

Introduce a new server-side service to read and record platform notifications
targeted at the webapp.

- Add payload schema (v1) using zod and typed PayloadV1.
- Define PlatformNotificationWithPayload type and scope priority map.
- Implement getActivePlatformNotifications to:
  - query active WEBAPP notifications with scope/org/project/user filters,
  - include user interactions and validate payloads,
  - filter dismissed items, compute unreadCount, and return sorted results.
- Add helper functions:
  - findInteraction to match global/org interactions,
  - compareNotifications to sort by scope, priority, then recency.
- Implement upsertInteraction to create or update platform notification
  interactions, handling GLOBAL-scoped interactions per organization.

These changes centralize notification read/write logic, enforce payload
validation, and provide deterministic ordering and unread counts for the
webapp UI.

Author: 0ski